The present invention relates to decontamination processes. In particular, it relates to chemical decontamination of the surfaces of bodies contaminated by radioactive species.
Plant and equipment used in the processing, handling, transport and storage of radioactive materials, eg as in the nuclear materials processing or reprocessing industry, becomes contaminated during routine operations. Where contamination levels result in an excessive radiation hazard to plant operators a process for reducing the potential radiation hazard by decontamination is required to be employed. Also, redundant plants or components require decontamination during decommissioning operations to minimise the volume of bulk material requiring disposal as radioactive waste categorised as intermediate level waste (ILW). Removal of the radioactive contaminants from such plants and components in minimum volume allows the bulk material to be recategorised as low level waste (LLW) which category is considered much less hazardous and is therefore much cheaper to store, transport and dispose of than ILW.
There are many well established methods employed in the nuclear industry which use chemical formulations based upon strong mineral acids for the dissolution of radioactive contaminants contained on the surfaces of bodies to be decontaminated. Examples of such formulations are based upon nitric acid, phosphoric acid and hydrochloric acid. These formulations, which may also include oxidising agents and/or fluoride ions, eg applied in the form of sodium fluoride or hydrofluoric acid, can achieve rapid dissolution of metallic substrates, such as stainless steel, used as plant structures and components. These formulations are therefore used as solvents to dissolve a surface layer of the contaminated body or substrate, the surface contaminants being extracted by the solvent as part of the surface layer removal.
Such dissolution processes are very aggressive and cannot achieve uniform substrate surface removal because of preferential attack of stress cracks, island corrosion sites and other substrate surface non-uniformities. Corrosive and toxic fumes are also frequently released during the dissolution reaction requiring the provision of complicated and expensive off-gas scrubber systems.
A major disadvantage of these strong mineral acid decontamination processes is that the acid becomes a contaminated waste stream requiring further processing. Prior to neutralisation and discharge to the environment, it is necessary to remove, eg by floc precipitation and/or ion exchange, the contaminant species which may subsequently be immobilised and encapsulated in a solid matrix as a solid ILW. This consequently gives rise to considerable volumes of radioactive secondary wastes and liquid effluents.
Other milder decontamination processes, typically employed in nuclear reactor cooling circuit decontamination, have reduced the problems arising from the volume of the secondary waste in the aforementioned processes. Formulations of organic reagents such as vanadous formate, oxalic/citric acid used in successive cycles, with pretreatment to oxidise the contaminants, can solubilise the contaminant activation products and transport them for entrainment on organic ion exchange resins, where the organic reagent is simultaneously regenerated for re-use. Such processes offer a significant reduction in liquid effluent arising over the strong mineral acid processes, although they do show two notable disadvantages. Firstly, the spent organic ion exchange resin produced is a relatively unstable ILW. Secondly, the organic reagents cannot solubilise actinides and certain fission products or achieve a kinetically favourable dissolution of stainless steel bodies beneath the superficial oxide layer formed on such bodies. These disadvantages render such mild processes ineffective for the decontamination of many nuclear plant structures and components.
Recent developments in the field of nuclear plant decontamination have involved the use of tetrafluoroboric acid, HBF.sub.4. This acid is a well known solvent for metals and has been used in the metal finishing industry for many years. It is a comparatively inexpensive mineral acid produced, for example, as a by-product of the aluminium extraction industry. HBF.sub.4 can achieve a maximum capacity for dissolving iron of 220 grammes per liter, which compares with a capacity of 20 grammes per liter for dissolution of iron by concentrated HNO.sub.3. This demonstrates a clear advantage in using HBF.sub.4 for the dissolution of metals to provide surface decontamination. Furthermore, HBF.sub.4 achieves a uniform attack of metal surfaces without exhibiting preferential dissolution of stress cracks or island corrosion sites and the stability of the reaction products ensures minimal toxic gases are released during metal dissolution. The large capacity for iron dissolution and the rapid reaction kinetics of the dissolution process allow low concentrations of HBF.sub.4 to be used in the decontamination process and allow stoichiometric control of the metal dissolution process such that corrosion of the components being decontaminated can be maintained at a practical minimum. This stoichiometric control of the dissolution process is an important feature of HBF.sub.4 decontamination when structures or components are to be returned to service after decontamination as it is possible to demonstrate that the structural integrity of the bulk component or plant has not been compromised during the decontamination process.
A minimal liquid effluent process for decontamination of nuclear plant using HBF.sub.4 has been described in the prior art. HBF.sub.4 which has been used in the decontamination process is passed to an electrochemical cell where it is regenerated for re-use. Metal contaminants are also removed from the acid in the cell. The process is dependent on balancing the rate of dissolution of iron with the electrochemical regeneration process. The optimum iron dissolution capacity is 70 to 72 grammes per liter which is much less than the maximum possible capacity of 220 grammes per liter. A major disadvantage of this known process is that the decontamination liquor always contains a quantity of dissolved metal and radioactive contaminants. The presence of these contaminants which are not recovered electrochemically can present a serious criticality and radiation dose hazard.
Another known process is described in SU 1783585A1. This employs two different decontaminants in successive decontamination stages. In a first stage HBF.sub.4 solution is used to remove an initial layer 1 to 10 .mu.m thick from the surface of the body to be decontaminated. Oxalic acid is used to regenerate HBF.sub.4 from the liquor containing dissolved material from the body surface. This HBF.sub.4 is re-applied to the body surface. However, the re-applied HBF.sub.4 contains oxalic acid and oxalates of radionuclides and these oxalates are caused to plate out on the body surface. In a second decontamination stage another decontaminant comprising H.sub.2 SO.sub.4 is applied to the body surface which removes a further layer of the surface including the plated oxalates.
The disadvantages of the process described in SU 1783585A1 are that radionuclides initially removed from the body surface in the first stage are subsequently re-plated on the body surface and that the decontaminant used in the second stage is not intended to be re-generated and re-applied which means that the liquid effluent from the second stage is substantial and requires further treatment to remove radionuclides therefrom before the effluent can be safely discharged.